1. Field of the Invention
The present invention relates to a seam-coated welded tinplate can, which is excellent in the adhesion of a protecting resin coating to the welded seam and in which the anti-corrosive action of the coated seam can be maintained durably even after processing such as flanging, double seaming, beading and necking.
2. Description of the Prior Art
A tin-plated steel sheet, that is, a tinplate sheet, is a material excellent in the corrosion resistance, the coating adhesion and the processability and has been used from old as a can blank. Tinplate cans are ordinarily formed according to a method comprising forming a can blank into a cylinder and bonding both the side edges of the cylinder by lap seam soldering, lock seam soldering or combination thereof while using a solder. According to this can forming process, a considerable area is necessary for the seam portion, and the process therefore includes a problem in connection with saving of the resource. Furthermore, in case of food cans, bad influences are brought about by soldering. Therefore, a can forming process not using a solder has been desired in the art.
Instead of soldered cans, seamless cans prepared by draw-ironing have been used in certain fields. However, deformation of the side wall by application of pressure is extreme in seamless cans, and hence, they cannot be conveniently used as vacuum cans, that is, cans of the type where canned contents are subjected to retort sterilization. As another instance that can be used instead of the soldered can, there is known a welded can formed by lap-bonding both the side edges of a can body by welding. The area of the lap seam portion in such welded can is much smaller than in the soldered can and this lap bonding method is advantageous in that a particular adhesive, for example, a solder, need not be used. However, known welded seam cans of tinplate are still insufficient in the appearance of the seam, the corrosion resistance and the coating adhesion.
In the manufacture of welded seam cans, lap portions prepared by forming a can blank into a cylinder are subjected to electric resistance welding by passing the lap portions through between a pair of upper and lower electrode rollers or passing the lap portions through between a pair of upper and lower electrode rollers via an electrode wire. However, when tinplate is used as the can blank, a tin plating layer is fused at the welding step and fused tin is transferred to the electrode member or is scattered in the form of fume, with the result that the protecting effect of the tin plating layer is lost and black or blue iron oxide is readily formed on the surface. A film of such iron oxide comes to have a thickness of 700 to 3000 A.
This iron oxide is brittle and porous. Accordingly, if a protecting resin coating is applied to this iron oxide, adhesion is poor and dissolution of iron into the contents or corrosion of the seam is hardly prevented. Such defects of welded cans, that is, poor adhesion to the coating and poor corrosion resistance on the inner side seam, are made more serious when welded seams are subjected to processing necessary for the manufacture of cans, such as flanging, double seaming, necking or beading or when cans are subjected to a severe hydrothermal treatment such as sterilization after packing of the contents.